1. Field of the Invention
The present invention relates to a head drum for a magnetic recording/reproducing apparatus and, more particularly, to a head drum for a magnetic recording/reproducing apparatus having an improved hydraulic bearing system.
2. Description of the Related Art
A head drum is used in a magnetic recording/reproducing apparatus for recording data on magnetic recording tape or reproducing the recorded data from the magnetic recording tape. The head drum comprises two opposing drum members, one of the two members rotating with respect to the other, containing a head mounted therein. The magnetic recording tape travels in contact with the outer circumference of the head drum, and the rotatable drum member rotates to detect an electrical signal from the tape. Such a head is depicted in FIG. 1.
In FIG. 1, a lower drum 70 is fixed to the lower portion of a shaft 60. An upper drum 80 is installed at the upper portion of the shaft 60 and is rotatable with respect thereto. Thus, the upper drum 80 comprises a body 84 the outer circumference of which comes into contact with magnetic recording tape, and a bush 85 press-fitted into the center of body 84. The top end surface of shaft 60 is in contact with a thrust hydraulic bearing 90 fixed to the upper drum 80 by a bolt 81. Radial hydraulic bearings 66 are formed on the outer circumference of shaft 60 at locations coming into contact with upper drum 80. The structures of the hydraulic bearings 90 and 66 are schematically shown in FIGS. 2, 3 and 4. FIG. 2 is a bottom view of the thrust hydraulic bearing 90. FIG. 3 is a sectional view of FIG. 2 cut along the line III--III thereof. FIG. 4 is an enlarged view of the radial hydraulic bearing 66 extracted from the head drum 9 shown in FIG. 1.
Referring to FIGS. 2, 3 and 4, a plurality of grooves 91 are provided at the center portion of thrust hydraulic bearing 90 coming into contact with the top end surface of the shaft 60. A lubricant such as air or oil is introduced into the grooves. Grooves 67 are formed on the outer circumference of the shaft 60 at locations coming into contact with upper drum 80 so as to constitute a radial hydraulic bearing 66. The lubricant is also introduced into grooves 67.
In the head drum constructed as above, as the upper drum 80 rotates with respect to the shaft 60, a frictional resistance produced by a force acting in the radial direction of shaft 60 is generated between the upper drum 80 and the outer circumference of the shaft 60 at locations coming into contact with upper drum 80. Further, frictional resistance acting in the axial direction of the shaft 60 is created due to the weight of the upper drum 80. The frictional resistances generated by the forces acting in the axial direction and radial direction decrease due to the thrust hydraulic bearing 90 and the radial hydraulic bearing 66 so that the upper drum 80 rotates smoothly with respect to the shaft 60. The radial hydraulic bearing 66 may be formed on the inner circumference of the bush 85 instead of on the outer circumference of the shaft 60, thereby obtaining the same effect as that of the head drum 9.
In order to reduce the frictional resistance between the upper drum 80 and the shaft 60 created due to the rotation of the upper drum 80 and thereby to smoothly rotate the upper drum 80, the conventional head drums separately comprise radial hydraulic bearing 66 and thrust hydraulic bearing 90, which thus involves the following problems:
First, the radial hydraulic bearing 66 requires the grooves formed on the outer circumference of the shaft 60 or on the inner circumference of the bush 85. However, it is very complicated to form the grooves on the outer circumference of the shaft 60 or on the inner circumference of the bush 85. Further, in assembling the head drums, there must be strictly maintained a parallelism between the thrust hydraulic bearing 90 and the end surface of the shaft 60 coming into contact therewith, so that the assembling process is slow and unreliable.